This invention relates to a process for producing and then rapidly recovering magnesium hydroxide from an aqueous liquid, such as sea water and the like, which contains magnesium ion.
The invention has particular application to the extraction of metallic magnesium from sea water. As is well known, magnesium is the second-most abundant metal in sea water. Sea water contains about 0.13% magnesium in the form of magnesium ion. This is considered a virtually limitless source of magnesium. An electrolytic process is used to obtain metallic magnesium from sea water. In this process, sea water is pumped into large settling tanks where it is mixed with lime (calcium oxide). The lime converts the magnesium ion into insoluble magnesium hydroxide which is separated from the liquid. The hydroxide is then treated with hydrochloric acid to produce a magnesium chloride solution. The water is evaporated, and the dry magnesium chloride is fed to electrolytic cells which break it up into metallic magnesium and chlorine.
Heretofore, techniques developed for separating insoluble magnesium hydroxide from sea water have generally involved the use of large, slow-acting settling chambers. Principal difficulties with prior art separating techniques and apparatus include the prolonged periods of time needed to effect settling or separating and the relatively large capacity and expense required for the separation equipment. For example, in a typical commercial process, magnesium hydroxide is separated from sea water by sedimentation in "clarifiers" which are up to 300 feet in diameter. These clarifiers may be used to process 10,000 gallons of sea water per minute. This corresponds to a rate of 0.14 gallons per minute per square foot (g.p.m./sq. ft.) of clarifier area. Often, several clarifiers are operated in combination in order to process up to 25,000 gallons of sea water per minute. In these commercial processes, the underflow from the final clarifiers contains about 25% suspended solids, while the overflow frequently contains over 100 parts per million suspended solids.